I have had mirena (levonorgestrel) for almost 3yrs n haven't had any bleeding past the first month. I have started heavy spotting. What could cause this? Am I preganat

Not likely, but... Unexpected bleeding with a Mirena (levonorgestrel) iud should be checked with a pregnancy test. It is most likely due to declining Progesterone levels from the device. Protection from pregnancy should still be good for the full 5 years despite the bleeding.